Most of the boiler failures can be attributed to low water conditions. For safe boiler operations this must be kept under check. Low-water level conditions in the boiler occur when the water in the boiler drum falls below the low water level mark. Such conditions have the potential to severely damage the boiler. The furnace temperatures usually exceed 1,800°F (980°C). The strength of steel drops sharply at temperatures above 800°F (480°C). Hence, a Low-water condition has the potential to melt the steel tubes.
Controlling the boiler drum level is complicated as the "water level" in a steam drum is actually a fairly unstable mixture of water and steam bubbles that shrinks and swells as the pressure changes. This is because of the fact that as the steam demand increases, the pressure in the boiler drops causing the water level to rise. Also when the fresh water is added from the feedtank it cools the drum water, leading to collapse of the steam bubbles and an immediate drop in the water level. To overcome this shrink and swell effect in boiler one may opt for a 3 level drum control.
Both operator and equipment error result in low water conditions. Some of the common causes include:
- Feedwater pump failure
- Control valve failure
- Loss of water to the deaerator or make-up water system
- Drum level controller failure
- Drum level controller inadvertently left in "manual" position
- Loss of plant air pressure to the control valve actuator
- Safety valve lifting
- Large, sudden change in steam load
For safe boiler operations it must be ensured first that the low water level trips are functional at all times. In case a trip occurs, the operator must check for the root cause to avoid unsafe operating conditions.